The present invention relates to a method of controlling a spark-ignition engine including an ignition plug that is spark-ignitable to a mixture gas inside a cylinder formed in an engine body.
Conventionally, a method of controlling an engine, to suppress a pre-ignition that is caused by a phenomenon wherein a mixture gas self-ignites before the normal start timing of combustion due to spark-ignition, has been developed. If the pre-ignition occurs, the combustion dramatically speeds up according to the advanced start of combustion, and as a result, engine noise and vibration increase and may cause damage on, for example, a piston. Therefore, it is necessary to more surely avoid the occurrence of pre-ignition.
For example, JP2001-159348A discloses a method of controlling an engine in which a close timing of an intake valve is changed so that an effective compression ratio does not exceed a predetermined maximum value, so as to avoid a pre-ignition.
The maximum value of the effective compression ratio is a threshold where the pre-ignition occurs if the effective compression ratio exceeds said threshold. The maximum value of the effective compression ratio depends on many environmental conditions, and therefore the change of the maximum value according to changes in the environmental conditions is difficult to grasp entirely and accurately. If the change of the maximum value is not appropriately grasped, the effective compression ratio may exceed the actual maximum value and pre-ignition may occur.
The present invention is made in view of the above situation, and further surely avoids an occurrence of pre-ignition.